The invention relates to a system for controlling a hydraulic propulsion drive, having a closed hydraulic circuit which has a variable displacement pump driven by a driving motor, activation valves, a hydraulic motor driving the vehicle, and a flush valve.
Systems of this type are part of the prior art. The flush valve is used in this case in order to renew the hydraulic fluid from the closed circuit during operation. This renewing is intended to prevent the hydraulic fluid in the closed circuit from becoming increasingly contaminated by particles due to abrasion. Furthermore, excessively high temperatures of the hydraulic fluid in the closed circuit are thereby avoided. In the case of these known systems, the flush valve is hydraulically controlled. The system of a changeover slide valve has the effect that it is always the line having the lower operating pressure (return line) which is connected to the flush valve. At a certain value, the pressure prevailing in this low pressure line then automatically opens the flush valve. It is also already known to combine the flush valve with an orifice connected upstream, or just to use an orifice. The version having a pressure-operated flush valve and orifice has the advantage that the quantity to be flushed can be set more readily. This is because the quantity of hydraulic fluid flushed out results from the diameter of the orifice and the set value of the low pressure. The use of just an orifice is particularly cost effective.
Common to these known solutions is the fact that the flushing procedure is always determined as a function of the hydraulic fluid pressure prevailing in the low pressure line of the closed hydraulic circuit. In this case, however, flushing can not always be done in accordance with requirements because the flushing procedure is triggered and controlled independently of the contamination and the temperature of the hydraulic fluid.
Therefore, the principal object of this invention is to control the flushing procedure in such a manner that it fulfills the actual requirements, i.e. the extent of contamination and of heating of the hydraulic fluid, with the use of an electrically controlled flushing valve.
The invention relates to a system for controlling a hydraulic propulsion drive, having a closed hydraulic circuit which has a variable displacement pump driven by a driving motor, activation valves, a hydraulic motor driving the vehicle, and an electrically controlled flush valve.
The electrical control makes provision for the prerequisite of it being possible for the flushing procedure to be triggered and controlled as a function of other essential influencing variables, rather than just by the pressure of the hydraulic fluid in the low pressure line of the closed hydraulic circuit. The advantages of integrating it into a modern, versatile, automatic control are very substantial.
Particular advantages result if the flush valve is controlled as a function of the vehicle speed, the rotational speed of the driving motor and/or of the temperature of the hydraulic fluid in the variable displacement pump in the closed hydraulic circuit. Specifically, the flushing takes place if the vehicle speed, the rotational speed of the driving motor and/or the temperature of the hydraulic fluid in the variable displacement pump are high. The flushing procedure is always only initiated and carried out if the particular operating conditions of the vehicle permit and require it.
The closed hydraulic circuit needs to be flushed, in particular at high traveling speed, because low pressure in this case means that only a smaller amount of hydraulic fluid leaks out of the closed circuit. In addition, at high traveling speed the variable displacement pump of the closed hydraulic circuit has to convey a maximum amount of hydraulic fluid, the frictional losses of the power train components and the flow being at a maximum, and most lost heat is being produced.
Since the flush valve is electrically controlled, the dependence on the numerous operating parameters can be realized with particular advantages in an automatic control.
In the case of a vehicle having a combustion engine as the driving motor for the variable displacement pump of the closed hydraulic circuit, the automatic control can be implemented with a digital computer which is connected on the input side to sensors. The sensors are situated on the vehicle and give information about the vehicle speed, the rotational speed of the combustion engine and/or the temperature of the hydraulic fluids in the variable displacement pump of the closed hydraulic circuit, and which controls the flush valve after processing this data.
The digital computer evaluates the signals supplied by the sensors and concerning the operating stage of the vehicle. Via its software, the computer switches the flush valve on if the vehicle speed or the rotational speed of the combustion engine or the temperature of the hydraulic fluid in the variable displacement pump are high. Conversely, when traveling slowly and at low rotational speed of the combustion engine, the flushing procedure is automatically terminated. In practice that means only permitting the flushing procedure at, a relatively high traveling speed, or when critical temperature limits, are exceeded, or at high rotational speeds of the combustion engine, because the feed pump is then able to convey a sufficient amount of hydraulic fluids.